MediumCVE-2026-116579

Crossbario Autobahn-python: Improper Handling of Highly Compressed Data (Data Amplification) (CVE-2026-116579)

Published 26.10.2026Last modified 27.10.2026Sources: OSV, Hersteller, CVE.org

What is affected, what to do

Update autobahn to version 26.10.1.

Affected: Crossbario Autobahn-python gte 26.7.1, < 26.10.1..*

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

5.9

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: Vendor (CNA), CVSS v3

5.9

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: OSV, Hersteller, CVE.org

0.4 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 36.7

Show all metrics

Severity: Medium
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 70.29 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: no public PoC known
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 5.9 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 5.9
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.4 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 36.7; High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 26.10.2026
CISA assessment (SSVC): Exploitation: none known · not automatable · Technical impact: partial; Source: CISA Vulnrichment, as of 27 October 2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-409, CWE-770

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Summary

permessage-brotli inflates each frame fully before checking maxMessagePayloadSize, so one 421-byte frame makes the server allocate about 545 MiB even with a 1 MiB message limit (brotli expands about 1,000,000:1). Any unauthenticated client can exhaust server memory this way on servers that enable permessage-brotli. This bypasses the GHSA-hxp9-w8x3-p566 fix.

Details

In 26.7.1 the receive path passes the remaining maxMessagePayloadSize budget to decompress_message_data(..., max_output_len=...). deflate and bzip2 stop at that budget. PerMessageBrotli.decompress_message_data (compress_brotli.py) instead calls self._decompressor.process(data) with no limit and compares the length afterwards, so the whole frame is inflated first.

The code comment and changelog say this is still "bounded on the wire by maxFramePayloadSize", but brotli reaches about 1,000,000:1 (841 bytes inflate to 1 GiB), so that bound means little. A 421-byte frame against a 1 MiB budget peaks at 545 MiB before the 1009 rejection.

An autobahn asyncio server on 127.0.0.1 accepting permessage-brotli, with maxFramePayloadSize=64 KiB and maxMessagePayloadSize=1 MiB, received one small brotli frame from an autobahn client and peaked at 545 MiB (baseline 28 MiB). A few concurrent connections can exhaust memory.

brotli ≥ 1.2.0 has Decompressor.process(data, output_buffer_limit=N), so the backend can stop at max_output_len like deflate and bzip2 (and the extra's floor can be raised to brotli>=1.2.0).

Tested on autobahn 26.7.1 (latest) with brotli 1.2.0; compress_brotli.py on master is unchanged. This affects servers that enable permessage-brotli (opt-in, needs the compress extra).

Proof of concept

How to run the PoC

python3 -m venv venv && . venv/bin/activate
pip install autobahn==26.7.1 brotli==1.2.0
python3 poc_autobahn.py

# PoC: autobahn 26.7.1 permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked.
import brotli, resource
from autobahn.websocket.compress import PerMessageBrotli

c = brotli.Compressor()  # build a 256 MiB-of-zeros frame without holding the plaintext
frame = b"".join(c.process(b"\0" * (1 << 20)) for _ in range(256)) + c.finish()
pmce = PerMessageBrotli(is_server=True, server_no_context_takeover=False, client_no_context_takeover=False)
pmce.start_decompress_message()
print(len(frame), "bytes on the wire; peak RSS before:", resource.getrusage(resource.RUSAGE_SELF).ru_maxrss >> 10, "MiB")
try:
    pmce.decompress_message_data(frame, max_output_len=1 << 20)  # remaining budget: 1 MiB
except Exception as e:
    print(type(e).__name__, "| peak RSS after:", resource.getrusage(resource.RUSAGE_SELF).ru_maxrss >> 10, "MiB")

Output:

421 bytes on the wire; peak RSS before: 36 MiB
PayloadExceededError | peak RSS after: 545 MiB

Impact

An unauthenticated WebSocket client can make a server that enabled permessage-brotli allocate hundreds of MB or more per tiny frame, regardless of the configured maxMessagePayloadSize. That is the memory-exhaustion DoS GHSA-hxp9-w8x3-p566 was meant to prevent.

Credit goes to Adrian Korwel.

Source: OSV (Open Source Vulnerabilities), GitHub Security Advisories, NVD (NIST), cvelistv5

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
crossbario	autobahn-python	`≥ 26.7.1, < 26.10.1`	—
pip	autobahn	`>= 26.7.1, < 26.10.1`	`26.10.1`
pypi	autobahn	`26.7.1`	`26.10.1`

Weakness classes (CWE)

CWE-409Improper Handling of Highly Compressed Data (Data Amplification)
Base
The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.
Entry at MITRE CWE-409
CWE-770Allocation of Resources Without Limits or Throttling
Base
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Entry at MITRE CWE-770

References & sources

github.comcrossbario/autobahn-python/commit/a41442bbee4f6126a8ce1bb892840…x_refsource_MISC, web
github.comcrossbario/autobahn-python/pull/1987x_refsource_MISC, web
github.comcrossbario/autobahn-python/releases/tag/v26_10_1x_refsource_MISC, web
github.comcrossbario/autobahn-python/security/advisories/GHSA-pjh6-c8mg-3…x_refsource_CONFIRM, web
NVD (NIST)vuln/detail/CVE-2026-116579advisory
github.comcrossbario/autobahn-pythonpackage

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

27.10.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: none known, automatable: no

Show the original change

Individual changes of this event as published by NVD
Type	Action	Before	After
SSVC assessment	added	—	{"id":"CVE-2026-116579","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"no"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-10-27T13:41:50.957164Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

26.10.2026ReceivedGitHub

CVSS V3.1: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Weakness type: CWE-409, CWE-770

Description added or changed

New references:

Show 9 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Description	added	—	Autobahn Python is a WebSocket and WAMP implementation for Python that supports Twisted and asyncio. From 26.7.1 until 26.10.1, PerMessageBrotli.decompress_message_data in compress_brotli.py calls the Brotli decompressor without an output limit and checks maxMessagePayloadSize only after a complete frame has been inflated. An unauthenticated WebSocket client can send a small, highly compressed frame that allocates hundreds of MiB before rejection, causing memory-exhaustion denial of service on servers that enable the opt-in permessage-brotli feature through the compress extra. Deflate and bzip2 already enforce the remaining output budget and are outside this residual vulnerability. This issue is fixed in version 26.10.1.
CVSS	added	—	AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness type (CWE)	added	—	CWE-409
Weakness type (CWE)	added	—	CWE-770
Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/116xxx/CVE-2026-116579.json">CVE-2026-116579</a>
Reference	added	—	https://github.com/crossbario/autobahn-python/commit/a41442bbee4f6126a8ce1bb892840124c24d16f1
Reference	added	—	https://github.com/crossbario/autobahn-python/pull/1987
Reference	added	—	https://github.com/crossbario/autobahn-python/releases/tag/v26_10_1
Reference	added	—	https://github.com/crossbario/autobahn-python/security/advisories/GHSA-pjh6-c8mg-3x7m

Source identifier: security-advisories@github.com

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